ci: add Gitea Actions gate (make gen/check/lint on push/PR)

- .gitea/workflows/lint.yml: on push + pull_request runs make gen, then
  git diff --exit-code (gen-drift check), then make check, then make lint
- requires a registered act_runner (ubuntu-latest label) to execute
- docs: AGENTS.md Quick facts 'no CI' -> CI bullet (runner required, gates
  still run locally); DEV.md stub-harness note clarifies CI is static-gates
  only; AGENT_TODO Done entry + Next item for runner registration
- verified locally: gen idempotent (0 gen-managed files changed), check OK,
  lint 0 FAIL / 0 WARN

.gitea/workflows/lint.yml

@@ -0,0 +1,18 @@
+name: gates
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  consistency-and-conventions:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run the repo gates (gen drift, check, lint)
+        run: |
+          set -e
+          make gen
+          git diff --exit-code
+          make check
+          make lint